Surface defect measurements are important in several industries. In semiconductor manufacturing, surface defect measurements are made during final inspection by the slice vendor, at incoming inspection by the circuit fabricator and before certain key operations. The principal requirements of the inspection systems are a low defect diameter detection threshold, good repeatability, high throughput and that they add very few particles of critical diameter. These functional requirements lead to sometimes conflicting design requirements for the optical measuring subsystems. We examine the key design requirements for future inspection equipment, and compare approaches used in commercially available equipment.